1. Field of the Invention
This invention relates generally to the field of integrated circuit design and, more particularly, to the design of temperature measurement circuits.
2. Description of the Related Art
Many digital systems, especially those that include high-performance, high-speed circuits, are prone to operational variances due to temperature effects. Devices that monitor temperature and voltage are often included as part of such systems in order to maintain the integrity of the system components. Personal computers (PC), signal processors and high-speed graphics adapters, among others, typically benefit from such temperature monitoring circuits. For example, a central processor unit (CPU) that typically “runs hot” as its operating temperature reaches high levels may require a temperature sensor in the PC to insure that it doesn't malfunction or break due to thermal problems.
Often, integrated circuit (IC) solutions designed to measure temperature in a system will monitor the voltage across one or more PN-junctions, for example a diode or multiple diodes at different current densities to extract a temperature value. This method generally involves amplifying a small voltage generated on the diode(s), and then subtracting voltage from the amplified temperature-dependent voltage in order to center the amplified value for conversion by an analog-to-digital converter (ADC). In other words, temperature-to-digital conversion for IC-based temperature measuring solutions is often accomplished by measuring a difference in voltage across the terminals of typically identical diodes when different current densities are forced through the PN junctions of the diodes. The resulting change (ΔVBE) in the base-emitter voltage (VBE) between the diodes is generally proportional to temperature. (It should be noted that while VBE generally refers to a voltage across the base-emitter junction of a diode-connected transistor and not a voltage across a simple PN-junction diode, for the sake of simplicity, VBE is used herein to refer to the voltage developed across a PN-junction in general.) More specifically, VBE may be defined as a function of absolute temperature by the equation
                              V          BE                =                  η          ⁢                      kT            q                    ⁢          ln          ⁢                                    I              C                                      I              S                                                          (        1        )            where η is the ideality factor of the PN junction, k is Boltzman's constant, q is the charge of a single electron, T represents absolute temperature, Is represents saturation current and IC represents the collector current. A more efficient and precise method of obtaining ΔVBE is to supply the PN junction of a single diode with two separate and different currents in a predetermined ratio. Consequently, ΔVBE may be related to temperature by the equation
                              Δ          ⁢                                          ⁢                      V            BE                          =                  η          ⁢                      kT            q                    ⁢                      ln            ⁡                          (              N              )                                                          (        2        )            where N is a constant representing a pre-selected ratio of the two separate currents that are supplied to the PN junction of the diode.
In certain cases, for example when the diode or PN-junction—which is being monitored to obtain temperature measurements—is remotely coupled to the measurement system through a twisted pair of wires, the output temperature reading may artificially increase due to system noise. More specifically, Electromagnetic Interference (EMI) may modulate the diode voltage VBE, resulting in inaccurate temperature-readings, as the ADC configured in the temperature measurement system can typically not differentiate between a noise-induced temperature increase versus true temperature increase.
Other corresponding issues related to the prior art will become apparent to one skilled in the art after comparing such prior art with the present invention as described herein.